For light use and quick videos I tend to go for Corel Video Studio Pro. It's easy to use, and comparatively quick to get the hang of, so something of a middle stage between the commercial editing and Windows Movie Maker. It's also affordable by comparison to some of the bigger products. (I use X3, but if you want to try before you buy, there's a free trial of the next one, X4, available from Corel here)
For heavy editing, it would be Final Cut Pro, but that is more expensive and not so user friendly.
For heavy editing, it would be Final Cut Pro, but that is more expensive and not so user friendly.